NOx in the atmosphere causes photochemical smog and acid rain. For that reason, emission of NOx from mobile emission sources such as automobiles equipped with internal combustion engines such as gasoline engines and diesel engines, which is one of the NOx emission sources, has become a social problem. Concerning the emission amount of NOx, an investigation has been promoted toward tightening its regulation in future. Therefore, studies to develop a catalyst for exhaust gas purification are attracting attention.
As for removal of nitrogen oxides in an exhaust gas, various technologies have been provided, in particular, many technologies using a zeolite-based catalyst have been reported. For example, U.S. Pat. No. 4,961,917 discloses a method for removing nitrogen oxides by using iron or copper and zeolite (ZSM-20, β, Y type) as a catalyst and NH3 as a reducing agent at 250 to 600° C. In addition, U.S. Pat. No. 5,141,906 discloses an catalyst for exhaust gas purification, which is obtained by forming a wash-coat layer containing zeolite (ZMS-5) ion-exchanged with copper or cobalt on the wall surface of a honeycomb support made of ceramics, followed by heat treatment in a gas stream containing a sulfur compound. Here, as zeolite, those of ZSM-5, ZSM-20, type β, type Y, and the like are generally used.